Any idea what the rating of those fusible links are? Just curious since I have a 100A feeder connected to one of them now which I'm guessing it can't really support but I haven't blown it yet so I guess I'm not drawing anywhere near that amount.

Any idea what the rating of those fusible links are? Just curious since I have a 100A feeder connected to one of them now which I'm guessing it can't really support but I haven't blown it yet so I guess I'm not drawing anywhere near that amount.

Billy,
You'll never be able to pull more than 1/2 the battery's reserve rating long enough to blow a fuse with that same rating. For instance, a battery with a reserve capactity if 100Ah should be fused your main fuse should be 50A, no matter the amount of current your amps can pull. It was weird when i heard it but it makes sense and it's never done me wrong.

Billy,
You'll never be able to pull more than 1/2 the battery's reserve rating long enough to blow a fuse with that same rating. For instance, a battery with a reserve capactity if 100Ah should be fused your main fuse should be 50A, no matter the amount of current your amps can pull. It was weird when i heard it but it makes sense and it's never done me wrong.

Batteries are actually capable of putting out very high amounts of current. The thing is they can only output that high current for a very short amount of time. Cold cranking amps (CCA) is a rating you see on most batteries and that rating is the amount of current that the battery can put out at 0 degrees F for 30 seconds while maintaining a cell voltage of 1.2V (6 cells per battery would mean your battery never dips below 7.2V). The OEM BMW battery is rated for about 650 CCA so you can see that capability is there to put out way more current than any audio system would need for a very short amount of time. Now battery capacity is typically expressed with a unit of time. For example, a battery like an Odyssey PC1200 has a rated capacity of 70Ahr. Odyssey defines this as their 20 hour rating. So if required to run for 20 hours, the battery could put out 3.5A and maintain a minimum cell voltage of 1.2V.

Now since audio is transient that means the current draw is changing very rapidly. It is highly unlikely that you would ever draw the max rating of any amplifier all at once because of the transient nature of music. The only time it would be remotely possible is if you were playing pink noise or sine waves for long periods of time at very high volumes. With music material, the transient peaks are too short to cause an large demand for an extended period of time. The fuse probably would have enough time to blow even if there was an overcurrent condition due to a transient peak however, you probably wouldn't want to allow the amp to see that on a regular basis because you will eventually burn out the output devices. Typically the fuse will blow on a fault condition like a short or an amp failure which causes an overcurrent condition at the amplifier's power input.

I have also ran smaller than usual fuses on some high powered systems. In my last, the recommended size for my amps were 125A each amp of which I had 2. The main circuit breaker at my battery was 150A. It never did trip because the instantaneous demand was always a lot lower than the max fuse rating of the amp.

Still, even though it would take a fault condition to burn that fusible link in the OEM distribution block, it would be nice to know what it's rated for. I'm guessing its somewhere between 50 and 70A since the connector is made for a 6 or 8 AWG conductor.

Batteries are actually capable of putting out very high amounts of current. The thing is they can only output that high current for a very short amount of time. Cold cranking amps (CCA) is a rating you see on most batteries and that rating is the amount of current that the battery can put out at 0 degrees F for 30 seconds while maintaining a cell voltage of 1.2V (6 cells per battery would mean your battery never dips below 7.2V). The OEM BMW battery is rated for about 650 CCA so you can see that capability is there to put out way more current than any audio system would need for a very short amount of time. Now battery capacity is typically expressed with a unit of time. For example, a battery like an Odyssey PC1200 has a rated capacity of 70Ahr. Odyssey defines this as their 20 hour rating. So if required to run for 20 hours, the battery could put out 3.5A and maintain a minimum cell voltage of 1.2V.

Now since audio is transient that means the current draw is changing very rapidly. It is highly unlikely that you would ever draw the max rating of any amplifier all at once because of the transient nature of music. The only time it would be remotely possible is if you were playing pink noise or sine waves for long periods of time at very high volumes. With music material, the transient peaks are too short to cause an large demand for an extended period of time. The fuse probably would have enough time to blow even if there was an overcurrent condition due to a transient peak however, you probably wouldn't want to allow the amp to see that on a regular basis because you will eventually burn out the output devices. Typically the fuse will blow on a fault condition like a short or an amp failure which causes an overcurrent condition at the amplifier's power input.

I have also ran smaller than usual fuses on some high powered systems. In my last, the recommended size for my amps were 125A each amp of which I had 2. The main circuit breaker at my battery was 150A. It never did trip because the instantaneous demand was always a lot lower than the max fuse rating of the amp.

Still, even though it would take a fault condition to burn that fusible link in the OEM distribution block, it would be nice to know what it's rated for. I'm guessing its somewhere between 50 and 70A since the connector is made for a 6 or 8 AWG conductor.

Think about this, it takes twice the current rating of a fuse to pass through it for two seconds before it will blow. And that's a normal "fast blow" type fuse. A slow blow fuse needs to see double the current for even longer. That's why you fuse at half the reserve capacity. As you said, music is transient so there is no need to ever run fuses over 60A or so. I've never run a fuse larger than 60A and I've never blown one. Ever. And you know i've run some serious power. Plus it keeps your car safer since less current will have to flow to blow the main fuse.

Think about this, it takes twice the current rating of a fuse to pass through it for two seconds before it will blow. And that's a normal "fast blow" type fuse. A slow blow fuse needs to see double the current for even longer. That's why you fuse at half the reserve capacity. As you said, music is transient so there is no need to ever run fuses over 60A or so. I've never run a fuse larger than 60A and I've never blown one. Ever. And you know i've run some serious power. Plus it keeps your car safer since less current will have to flow to blow the main fuse.

Agreed. That's why I mentioned that it would be rare for the fuse to blow on a peak current draw of the amp. It will typically blow due to a fault condition. Even circuit breakers probably wouldn't trip on the peak draw condition even though they are supposed to be more sensitive to overcurrent conditions than a fuse. As far as how to size the fuse, I do not relate it directly to battery capacity since there is a time component involved in the capacity rating. I base it off estimated demand. Its not an absolute calculation but more of a feel thing. But I guess if your method has been working for you than you're probably doing ok. Like you said, there's no benefit to oversizing and undersizing doesn't hurt unless you're constantly blowing fuses. On the flip side of that, slightly oversizing the fuse doesn't hurt either as long as your amp feeder is sized to match the fuse size and your amp has an internal fuse because a fault condition would still cause a high enough current draw to blow the fuse. You just want to make sure you're not going to burn the wire before the fuse blows.

After almost exactly a year to the day that I started this HiFi harness project, I finally have this sucker ready to sell today...

Very nice

BTW, what pin are you pulling the +12V from for the remote turn on relay. I think I want to go back and do this. Right now my remote is a direct tap off the OEM remote. Actually what I want to do is make it so that the amp doesn't turn on when the door opens. I only want it to turn on when the ACC is on. Is that +12V constant or ACC?

BTW, what pin are you pulling the +12V from for the remote turn on relay. I think I want to go back and do this. Right now my remote is a direct tap off the OEM remote. Actually what I want to do is make it so that the amp doesn't turn on when the door opens. I only want it to turn on when the ACC is on. Is that +12V constant or ACC?

That's pin 5 and it is the 12V constant used to power the OEM amp... if you want ACC then you do not need a relay, use a 1A fuse to the 12V jack in front of the shifter.

and can you make some with male RCAs?
because then i would just hook up all those speaker wires/RCAs straight to the amp
no need for connectors

That wouldn't make any sense because then he would have to make 10 different versions of the harness to accommdate all the different lengths of interconnects people might need. What's so hard about plugging in an interconnect cable? The female RCA is a standard connector on the back of any aftermarket head unit.

What's so hard about plugging in an interconnect cable? The female RCA is a standard connector on the back of any aftermarket head unit.

never said it was hard
just makes sense to just plug all the ends straight into the amp
Vs plugging more stuff into the connector that then goes to amp
plus he wouldn't have to make 10 different versions
i mean he chose 6" for the speaker wires
what if you need more?
well then you'd have to extend it, same as if the RCA was male

never said it was hard
just makes sense to just plug all the ends straight into the amp
Vs plugging more stuff into the connector that then goes to amp
plus he wouldn't have to make 10 different versions
i mean he chose 6" for the speaker wires
what if you need more?
well then you'd have to extend it, same as if the RCA was male

I think that 99.9% of the RCA extension cables that you will find commercially pre-made will be male to male just because 100% of the aftermarket HU, LOCs, processors, line drivers and aftermarket amps have female RCAs.

The reason of those lengths is very simple... I don't know where the customer will install the aftermarket amp.